Up to now, outer layers of triple- and multi-layer particle board have been prepared using the same phenolic resin glues as in the center layers. Whilst the high reactivity of such 45-48% aqueous resins is of advantage in the center layers, it causes embrittlement of the surface in the outer layers. Moreover, owing to the high alkalinity of such resins required for rapidly thermosetting center-layer resins having customary solids contents and viscosities, alkali-rich dust is obtained when grinding such particle board. On incineration this dust destroys the furnaces.
In view of the above mentioned difficulties special phenolic resins for outer layers of particle board were developed several years ago. The disadvantages were removed by lowering the alkali content of 45-48% resins from 9% and more and by lowering at the same time the formaldehyde content in order to reduce the reactivity and secure sufficient storage stability.
The phenol content in resins was thus raised by approx. 40%. Hence the price of the new outer-layer resins and consequently of the particle board prepared therefrom increased appreciably because the amount of solid resin applied to the particles could not be lowered despite the higher phenol content in the resin solutions in order not to deteriorate the particle board properties.
A method has now been discovered for the manufacture of multi-layer particle board allowing the use of phenolic resin binders for gluing particles in the outer layers. The new binders are employed in smaller amounts at comparably lower alkali contents.